1. Field of the Invention
The present invention relates to a mobile communication system, a radio network controller and a method.
2. Description of the Related Art
A W-CDMA system includes a radio access network (RAN) mainly responsible for performing radio access control and a core network (CN) responsible for performing mobility management, call control, service control and so on. The RAN includes a radio network controller (RNC) and a radio base station (NodeB). The CN includes a circuit switched (CS) domain and a packet switched (PS) domain. In general, the CS domain provides voice related services while the PS domain provides data communication related services. As nodes for serving mobile stations in the respective domains, a mobile-services switching center (MSC) and a serving GPRS support node (SGSN) are provided in the CS domain and the PS domain, respectively. The MSC and the SGSN have interfaces to the RNC, and the interfaces between the RNC and the MSC/SGSN are referred to as Iu interfaces.
In a conventional prevailing network, a single exchange (MSC/SGSN) controls the RNC. In the case of multiple CN nodes (exchanges) within a domain, if the RNC can select one of the multiple CN nodes within the domain at the time of call connection, it would be possible to distribute load within the CN. Also, in the case of failure of a certain CN node in the domain, if another CN node is available, it would be possible to improve redundancy of the CN nodes without interrupting the call connection. From these viewpoints, 3GPP standard specification defines a scheme for enabling the RNC to determine a connecting CN node in a network where multiple CN nodes are included in a domain. This scheme is referred to as Iu-Flex. For reference, see 3GPP TS 23.236 V8.1.0 (2009-12) and 3GPP TS 25.331 V.8.10.0 (2010-03), for example.
In an Iu-Flex based network configuration, subscriber profiles for mobile stations are distributed and accommodated among multiple CN nodes. If call connections or location registration traffic is concentrated on one or several of the CN nodes due to some factors, these CN nodes would become congested and accordingly could not continue to operate normally. Also, the CN nodes within a pool area may be arranged to have different processing capabilities due to some reasons such as facility investment policies. In this case, a CN node having a relatively lower processing capability may be likely to fall into a congestion state and accordingly may not be able to continue normal operations. The term “pool area” used herein means an area to accommodate multiple CN nodes connected to a single RNC.
If a congested CN node receives a location registration request from a mobile station, the CN node would issue a TMSI and/or a P-TMSI including an NRI (Network Resource Identifier) having an invalid value to the mobile station. Thus, when the mobile station issues another location registration request at the next time, the NRI would indicate the invalid value in an RRC message (Initial Direct Transfer). Upon detecting that the NRI has the invalid value, the RNC selects a CN node from available CN nodes randomly and transmits a location registration request signal to the selected CN node. In this manner, the transmission of the location registration request signal to the congested CN node can be avoided, and the RNC can connect to other CN nodes. As a result, it is possible to redistribute load among the CN nodes.
In the above-mentioned manner, however, since the congested CN node transmits the invalid NRI value in response to receipt of the location registration request, the load redistribution would not be available until the next occasion of the location registration. In other words, the RNC would have to transmit the location registration request signal to the congested CN node at least once. For this reason, the above-mentioned method is disadvantageous in that the load on the congested CN node cannot be redistributed immediately. Also, the congested CN node has to set the invalid NRI value in the TMSI and/or P-TMSI and issue the TMSI and/or P-TMSI, which may increase processing load of the CN nodes.
In addition, if the mobile station attempts to initiate communication after the transmission of the TMSI and/or the P-TMSI having the invalid NRI value before completion of the next location registration, the NRI in a signal transmitted by the mobile station would be invalid. Since the RNC has no knowledge of the CN node accommodating that mobile station, the RNC would select the CN node randomly. For this reason, there is a higher likelihood that the signal may be transmitted to a non-congested ON node. On the other hand, a connection request would be issued to the CN node having no subscriber information for the mobile station, which leads to connection failure.
Furthermore, in the Iu-Flex scheme, even if the CN node is in the congestion state, a normal connection request from a mobile station, whose location has not been registered yet, would be transmitted to the congested CN node. This is because the location registration triggers the load redistribution in accordance with the above-mentioned load redistribution scheme.
Accordingly, the conventional load redistribution scheme has no immediate effect. If the processing capability approaches the limit and accordingly the load has to be reduced immediately, the conventional load redistribution cannot follow it.